The invention relates to an electrohydraulic steering system.
Electrohydraulic steering systems are intended predominantly to offer the user of an agricultural machine greater comfort or to make automatic procedures possible. These agricultural machines include for instance agricultural tractors and wheel loaders.
In German Patent Disclosure DE 102 57 130 A1, an electrohydraulic steering system of this generic type is disclosed in which pressure fluid is fed to a steering motor via a steering unit, as a function of a steering lock set via a user control element, such as a steering wheel or a joystick. The fundamental construction of this kind of steering unit, also known as Orbitrol, is disclosed for instance in the data sheet RD 14 365/07.03 published by the present Applicant. A steering unit of this kind essentially comprises a manually actuated servo valve of the rotary slide valve type, a rotor set (metering pump) operating on the gerotor principle, and the valves necessary for the hydraulic steering circuit. For increasing the steering quantity, parallel to the flow of pressure fluid fed to the steering motor via the steering unit, an additional volumetric flow of pressure fluid is fed into the steering motor via a steering valve. This kind of electrohydraulic steering system, in tractors, for instance, makes it possible to drive in unplanted wheel tracks, where the control is done via external signals, such as position signals of a GPS, and the steering quantity is set solely by way of the steering valve. In a so-called booster steering system, in addition to the volumetric flow of pressure fluid fed via the steering unit, a defined additional quantity of pressure fluid is fed to the steering motor via the steering valve, so that for a defined steering angle at the wheels of the machine, the number of turns of the steering wheel necessary is reduced. A power steering system can also be attained, in which in addition to the steering wheel, further user control elements, such as a joystick or potentiometer, make it easier for the user to steer the agricultural machine.
Since these agricultural machines are increasingly involved in traffic, and their permissible speeds are constantly increasing, particular attention must be paid to safe function of the electrohydraulic steering system.
For instance, if the steering valve fails—for instance because of a seized piston—then the regulating piston of the steering valve may no longer be able to return to its neutral position (blocking position). In that case, even though the user has actuated the user control element in the direction of reducing the steering lock, nevertheless an additional quantity of pressure fluid continues to be fed to the steering motor via the proportional valve, so that the user has no capability, or only a very restricted capability, of steering correction; the agricultural machine becomes uncontrollable. Although it is possible for the malfunction to be displayed using the travel monitoring of the steering valve that is present in the prior art, nevertheless the user cannot actively take any countermeasures to rescind the steering lock.